As known, front derailleurs are used to move the transmission chain from one crown gear of the crankset to another having a different diameter to carry out gearshifting, by changing the gear ratio.
Downward gearshifting is said when the chain passes from a crown gear having a larger diameter to a crown gear having a smaller diameter, and upward gearshifting is said when the chain moves from a crown gear having a smaller diameter to a crown gear having a larger diameter. Downward gearshifting therefore corresponds to the passage to a lower gear ratio and upward gearshifting corresponds to the passage to a higher gear ratio.
The movement of the front derailleur is obtained through an actuation device mounted on the handlebars so as to be easily maneuvered by the cyclist.
More specifically, in a mechanical gearshift, the front derailleur is moved between the crown gears of the crankset, upwards, by a traction action exerted by a normally sheathed inextensible control cable (commonly called Bowden cable), and downwards by the release of the traction of the control cable.
The derailleur comprises a chain guide (or cage) positioned above the transmission chain and a positioning mechanism of the chain guide that is fixed to the frame of the bicycle along the tube that connects the bottom bracket to the saddle (seat-tube).
The chain guide is formed from an inner plate and an outer plate facing one another and substantially parallel. The inner plate acts by thrusting on the chain to carry out the passage thereof from a crown having a small diameter to one having a larger diameter (upward gearshifting) and the outer plate acts by thrusting on the chain to carry out the passage thereof from a crown having a larger diameter to one having a smaller diameter (downward gearshifting).
The positioning mechanism of the chain guide is usually formed from a deformable articulated quadrilateral. Such an articulated quadrilateral comprises a fixed body (first side of the quadrilateral) fixedly connected to a strap for connection around the seat tube, by two connecting rods (inner connecting rod and outer connecting rod, which form another two sides of the quadrilateral) rotatably connected to the fixed body about two respective axes and by the body of the chain guide itself (which forms the fourth side and completes the articulated quadrilateral) rotatably connected to each of the two connecting rods about two further respective axes. The four axes are parallel to one another.
The outer connecting rod is hinged at the top to the fixed body about the first of the four axes and extends at the top beyond such an axis with an actuation arm to the end of which the control cable of the derailleur is connected. The control cable is usually connected to the end of the actuation arm laterally, so that the cable itself does not interfere with the positioning mechanism below.
When the control cable is pulled, the actuation arm is pulled and the outer connecting rod rotates about the first axis. The parallelogram is thus deformed, with the chain guide that is moved towards the outside of the bicycle to carry out gearshifting.
A return spring is usually active between the body of the chain guide and the inner connecting rod so as to counteract the tension of the control cable and translate the chain guide when the tension of the control cable is released.
In the actuation device, the control cable is actuated in traction or in release through winding and unwinding on a rotor element, commonly called cable-winding bush, the rotation of which is actuated by the cyclist with a suitable control lever, or with two control levers (a first lever for upward gearshifting and a second lever for downward gearshifting).
In any case, the actuation device must foresee for the cable-winding bush to be held in a number of predetermined angular positions, angularly spaced apart by predetermined indexing angles between a first and a last indexing position. This function is obtained with so-called indexers, many types of which are known in the field, variously active between the cable-winding bush and the fixed casing of the device.
A pair of mechanical end stops, and in particular an inner end stop and an outer end stop, limit the excursion of the derailleur to prevent it being able to shift the chain too much towards the frame or too much away from the frame (thus ensuring that the chain is not shifted beyond the crown gears).
Although the derailleur has a predetermined geometry, such as that summarily described above, its actual excursion following the actuation of the actuation device is a function of the actual mounting configuration of the derailleur and of the actuation device, said mounting configuration, although actuated within predetermined mounting tolerances, varying from one case to the next. For example, the actual force (or tension) transmitted by the control cable following the same rotation of the cable-winding bush depends (also) on the particular path followed by the control cable to reach the derailleur, on the way in which the control cable has been constrained to the actuation arm of the outer connecting rod and on many other factors linked to the installation of the derailleur and of the control device on the bicycle.
Clearly, such variations of the effective excursion of the derailleur are small in size, but sufficient to ensure that it is not possible to ensure that the total excursion of the derailleur (useful for carrying out all gearshifting operations) corresponds exactly to the angular excursion of the cable-winding bush between the first and the last indexing position.
The indexing positions and the relative controls are therefore sized so as the ensure correct gearshifting also in the most unfavorable mounting situation, implying that the angular distance between the first and the last indexing position corresponds to a useful stroke of the derailleur that is greater than what is actually necessary.
For this reason, the mechanical end stops of the derailleur are usually adjusted so that the outer end stop acts as a stop for the excursion of the derailleur when the cable-winding bush is held in the last indexing position, in other words in the indexing position reached after the last upward gearshifting.
In this way, it is possible to ensure for the cyclist that the last upward gearshifting operation, corresponding to the maximum tension of the control cable, is obtained and stably maintained.
The inner end stop is adjusted to ensure that the derailleur is positioned on the smallest toothed wheel of the crankset avoiding sliding of the transmission chain on the derailleur when the transmission chain is engaged on the largest toothed wheel of the rear cassette.
In this way, the excursion of the derailleur is stopped before the cable-winding bush reaches the first indexing position. However, the action exerted by the cyclist on the control in any case carries the cable-winding bush into the first indexing position, making the gearshifting stable. Indeed, in this situation (downward gearshifting) the rotation of the cable-winding bush towards the first indexing position causes a loosening of the control cable thus therefore does not offer any resistance to the rotation of the cable-winding bush.